1. Field of the Invention
The invention deals with the field of materials science. It relates to a nickel-base superalloy, in particular for the production of single-crystal components (SX alloy) or components with a directionally solidified microstructure (DS alloy), such as for example blades or vanes for gas turbines. However, the alloy according to the invention can also be used for conventionally cast components.
2. Discussion of Background
Nickel-base superalloys of this type are known. Single-crystal components made from these alloys have a very good material strength at high temperatures. As a result, by way of example the inlet temperature of gas turbines can be increased, so that the gas turbine becomes more efficient.
Nickel-base superalloys for single-crystal components, as are known from U.S. Pat. No. 4,643,782, EP 0 208 645 and U.S. Pat. No. 5,270,123, for this purpose contain solid-solution-strengthening alloying elements, for example Re, W, Mo, Co, Cr, and xcex3xe2x80x2-phase-forming elements, for example Al, Ta and Ti. The level of high-melting alloying elements (W, Mo, Re) in the basic matrix (austenitic xcex3 phase) increases continuously with the increase in the alloy loading temperature. For example, standard nickel-base superalloys for single crystals contain 6-8% of W, up to 6% of Re and up to 2% of Mo (details in % by weight). The alloys disclosed in the abovementioned documents have a high creep rupture strength, good LCF (Low Cycle Fatigue) and HCF (High Cycle Fatigue) properties and a high resistance to oxidation.
These known alloys were developed for aircraft turbines and were therefore optimized for short-term and medium-term use, i.e. the loading duration is designed for up to 20,000 hours. By contrast, industrial gas turbine components have to be designed for a loading duration of up to 75,000 hours.
By way of example, after a loading duration of 300 hours, the alloy CMSX-4 described in U.S. Pat. No. 4,643,782, when used in tests in a gas turbine at a temperature of over 1000xc2x0 C., reveals considerable coarsening of the xcex3xe2x80x2 phase, which is disadvantageously associated with an increase in the creep rate of the alloy.
It is therefore necessary to improve the oxidation resistance of the known alloys at very high temperatures.
A further problem of the known nickel-base superalloys, for example the alloys which are known from U.S. Pat. No. 5,435,861, consists in the fact that the castability for large components, e.g. gas turbine blades or vanes with a length of more than 80 mm, leaves something to be desired. It is extremely difficult to cast a perfect, relatively large directionally solidified single-crystal component from a nickel-base superalloy, since most of these components have defects, for example small-angle grain boundaries, freckles (i.e. defects caused by a sequence of uniaxially oriented grains with a high eutectic content), equiaxial scatter boundaries, microporosities, etc. These defects weaken the components at high temperatures, so that the desired service life or operating temperature of the turbine is not reached. However, since a perfectly cast single-crystal component is extremely expensive, industry tends to allow as many defects as possible without the service life or the operating temperature being impaired.
One of the most frequent defects is grain boundaries, which are particularly harmful to the high-temperature properties of the single-crystal article. Although small-angle grain boundaries have only a relatively small influence on the properties of small components, they are highly relevant with regard to the castability and oxidation characteristics at high temperatures in the case of large SX or DS components.
Grain boundaries are regions of high local disorder of the crystal lattice, since the adjacent grains abut one another in these regions and therefore there is a certain misorientation between the crystal lattices. The greater the misorientation, the greater the disorder, i.e. the larger the number of dislocations in the grain boundaries which are necessary for the two grains to fit together. This disorder is directly related to the performance of the material at high temperatures. It weakens the material when the temperature rises above the equicohesive temperature (=0.5xc3x97melting point in K).
This effect is known from GB 2 234 521 A. For example, in a conventional nickel-base single-crystal alloy, the fracture strength drops greatly at a test temperature of 871xc2x0 C. if the misorientation of the grains is greater than 6xc2x0. This was also recorded with single-crystal components with a directionally solidified microstructure, and consequently opinion has tended to be not to allow misorientations of greater than 6xc2x0.
It is also known from the abovementioned GB 2 234 521 A that enriching nickel-base superalloys with boron or carbon with directional solidification results in microstructures which have an equiaxial or prismatic grain structure. Carbon and boron strengthen the grain boundaries, since C and B cause the precipitation of carbides and borides at the grain boundaries, which are stable at high temperatures. Moreover, the presence of these elements in and along the grain boundaries reduces the diffusion process, which is a primary cause of grain boundary weakness. It is therefore possible to increase the misorientations to 10xc2x0 to 12xc2x0 yet still achieve good properties of the material at high temperatures. Particularly in the case of large single-crystal components made from nickel-base superalloys, however, these small-angle grain boundaries have an adverse effect on the properties.
Accordingly, one object of the invention is to avoid the abovementioned drawbacks. The invention is based on the object of developing a nickel-base superalloy which has an improved castability and a higher resistance to oxidation compared to known nickel-base superalloys. Moreover, this alloy is to be particularly suitable, for example, for large gas-turbine single-crystal components with a length of  greater than 80 mm.
According to the invention, this object is achieved through the fact that the nickel-base superalloy is characterized by the following chemical composition (details in % by weight):
7.7-8.3 Cr
5.0-5.25 Co
2.0-2.1 Mo
7.8-8.3 W
5.8-6.1 Ta
4.9-5.1 Al
1.3-1.4 Ti
0.11-0.15 Si
0.11-0.15 Hf
200-750 ppm C
50-400 ppm B
remainder nickel and production-related impurities.
The advantages of the invention consist in the fact that the alloy has very good casting properties and also has an improved resistance to oxidation at high temperatures compared to the previously known prior art.
It is particularly advantageous if the alloy has the following composition:
7.7-8.3 Cr
5.0-5.25 Co
2.0-2.1 Mo
7.8-8.3 W
5.8-6.1 Ta
4.9-5.1 Al
1.3-1.4 Ti
0.11-0.15 Si
0.11-0.15 Hf
200-300 ppm C
50-100 ppm B
remainder nickel and production-related impurities. This alloy is eminently suitable for the production of large single-crystal components, for example blades or vanes for gas turbines.